1. Field of the Invention
The present invention relates to heat transfer and cooling arrangements, and more particularly relates to high efficiency optimized heat transfer arrangements for high heat flux applications.
2. Art Background
The use of heat pipes in isothermal surface temperature heat transfer applications is well documented. Heat pipes were first identified in the 1940's as being highly efficient heat transfer mechanisms, and were later optimized during the late 1960's and 1970's in connection with heat transfer problems on board spacecrafts. The operation of a heat pipe as set out above is well known, wherein a working fluid contained within an evacuated vapor chamber fabricated of metallic or other high heat conduction material is caused to boil locally due to some heat flux sourced by a heated object in intimate contact with the heat pipe. The vaporized working fluid molecules then stream rapidly away from the heated region, and are condensed on cooler condenser surfaces some distance away. The condensed working fluid is then returned to the heated region via an applied wick along which the condensed working fluid moves under capillary action. A very rapid and efficient vaporization and condensation cycle is possible, increasing the heat flux and heat transfer capabilities of a heat pipe many orders of magnitude above simple conduction, even that of a high conductivity metal such as copper. A principal benefit of heat pipes is that surface temperatures of heated objects may be isothermally maintained to better than 1.degree. C. for reasonable power densities.
Presently, heat pipe assemblies consist of self-contained units including vapor chamber, condensing tubes, and heat exchange surfaces such as fins attached to the condenser tubes. As shown in FIG. 1A, a prior art heat pump assembly is shown having an evaporator 5 to which are attached condenser tubes 6, and fins 7. Evaporator 5 has a thermal wick 5a applied to it in a known manner. Similarly, a wick material 6a is applied to the interior surfaces of condenser tubes 6. Air is evacuated and an inert working fluid 15 is hermetically sealed within an interior volume within evaporator 5 and condenser tubes 6. The working fluid is caused to undergo an evaporation-condensation cycle according to heat input to evaporator 5 by a heated body. Thus fabricated, the heat pipe assembly is clamped, welded, or otherwise brought into intimate physical and thermal contact with electronic circuit module 3. In cases where clamping is used to attach heat pipe assembly to a heated object, a heat transfer medium must be applied at the interface of the heated object and the heat pipe assembly. The foregoing is true even in cases where objects, which under ordinary circumstances would be considered flat, in fact have sufficient surface topography variations to prevent adequate contact area, and thereby prevent efficient conduction from the heated object to the heat pipe evaporator. Accordingly, a thermal transmission medium 4 is disposed between the circuit module 3 and the evaporator 5. Electronic circuit elements 1 are attached to circuit module 3 with appropriate die-attach material 2, as known in the prior art. Thermal transmission medium 4 contributes a characteristic thermal impedance to the overall thermal impedance of the entire assembly, adding to the thermal impedances of the heat pipe evaporator 5 and heated circuit module 3 to be cooled by the heat pipe assembly. Referring briefly to FIG. 1B, a model thermal resistance network is illustrated for the heat pipe assembly shown in FIG. 1A. Thermal impedances contributed by component parts of the heat pipe including the circuit module 3 are seen to be additive, producing a total thermal impedance resisting the thermal gradient through which heat is to be removed by the condenser tubes 6 and fins 7.
The magnitude of the thermal impedance contributed by the thermal transmission medium 4 (e.g., heat transfer grease) has heretofore been inconsequential, especially in heat transfer applications where the heat transfer surface area is large, or where the heat transfer application is particularly efficient due to liquid cooling, low heat fluxes, etc. However, in recent years certain heat transfer applications have become increasingly critical, wherein heated objects to be cooled have become extremely small and yet produce high power densities and heat fluxes. Examples of such small but powerful objects might include extremely high current computation electronics, or perhaps military radar or other instrumentation systems. In such high-powered and compact arrangements, the thermal transmission medium 4 used to attach the heat-producing body (in this case, the electronic circuit module 3) contributes a thermal impedance which is no longer insignificant to overall thermal impedance of the system; rather, the thermal interface presented by thermal transmission medium 4 contributes a substantial if not controlling degree of thermal impedance, thereby limiting the heat transfer performance of the overall system. In critical applications where the thermal transmission medium 4 used to enhance contact surface area between a heated object and a heat pipe is a substantial or dominant contributor to the total thermal impedance of the system, it is desirable to eliminate such thermal interface entirely in order to enhance the heat transfer performance of the cooling system.
As will be set out in the following description, the present invention provides methods and apparatus for eliminating entirely the thermal tansmission medium interface in heat pipe heat transfer applications, thereby permitting the construction of extremely efficient optimized heat pipe heat transfer arrangements for size and power density crucial applications.